The invention relates to a parking brake for an automatic transmission.
DE 196 25 019 describes a parking brake for automatic transmission of motor vehicles which is loaded by moving a piston rod of a spring accumulator on whose end is mounted a pawl which engages in a parking gear. When the spring accumulator is loaded with hydraulic pressure, the piston rod moves in the opposite direction and the pawl is disengaged from the parking gear. The parking brake can be kept in a disengaged state by a holding magnet which has the task of preventing an undesired loading of the parking brake which can be the case, e.g. as result of failure of the hydraulic pressure when the internal combustion engine is disengaged during travel. The magnet must be provided in such a manner that the holding force of the magnet is stronger than the tension of the spring accumulator in order that the parking brake cannot be engaged when the hydraulic system fails. The magnet has a very high power consumption to provide the holding force. Thereby an undesired discharge of the vehicle battery could occur when the internal combustion engine is switched off and the magnet is supplied with current.
The problem on which this invention is based is to provide a parking brake in which when the hydraulic pressure is lost, the parking brake can remain disengaged for a greater time.
According to the invention, a parking brake, disengaged by the hydraulic pressure, is held by a locking system which is actuated by current supply of an electromagnet. While the electromagnet is supplied with current, one stud of the electromagnet moves in an axial direction against a spring tension. The stud has three areas:
a first cylindrical area of small diameter;
a second conical area which connects with each of the first and third areas; and
a third area of larger cylindrical diameter.
In an operative state of the magnet, the stud is retained by spring tension in one end position and the locking system elements are located in the first area of the stud which has a smaller diameter. The locking system elements are disengaged in this state and the piston of the spring accumulator can move. If the magnet is supplied with current, the stud moves out of its first end position to the second end position. The locking system elements are pressed outwardly by the second conical area and engage in a recess of a stationary part of the parking brake and connect the piston of the spring accumulator with the stationary part. In order to move the locking system elements, the magnet requires greater power consumption. If the stud is in its second end position, the locking system elements produce on the cylindrical area of the stud clear radial forces and the stud must be held in this position, by the magnet, only against the spring tension of the magnet. The power consumption of the magnet in this position is very small so that when the vehicle engine is stationary, this state can be maintained over a longer period of time without the vehicle battery discharging inadmissibly quickly. While the locking system elements positively connect the piston of the spring accumulator with a stationary part of the spring accumulator, the piston cannot be moved by the force of the spring of the spring accumulator due to the lack of hydraulic pressure and thus the pawl does not engage the parking gear. In this state, the parking brake remains effectively released. When the magnet is switched off, the spring of the magnet presses the stud back to its first initial position and the locking system elements are pressed inwardly so that the piston of the spring accumulator is again freely movable and the pawl can engage in the parking gear. In a preferred embodiment, the locking system elements consist of at least one or more ball(s) which move outwardly due to displacement of the stud in the piston of the spring accumulator and engage in a recess of the stationary part of the spring accumulator. By using balls as elements of the locking system, the power consumption of the magnet for moving the stud can again be reduced, sine due to the point of contact of the balls, a weak moving force is needed.